Back of foot hurts when walking (Achilles Tendon?)?

I'm not in any athletics just P.E. cl everyother day and we don't really do anything that is rigorous. Anyways today I was walking during ping periods and the back of my foot started to hurt and I thought it would go away but it just kept getting worse. I made it to my math cl and sat down. Once I sat down I felt no pain, I could move my foot and I didn't feel any pain. Then about 30 minutes later I had to walk to the cafeteria for lunch and I barely made it. The back of my foot really started hurting. It felt like the muscle was like grinding on my bone or something, I'm not sure but I think it's called the achilles tendon. What is weird is it just happened out of the blue and I didn't do any type of physical activity that was any different from what I usually do either. I was born with tight leg muscles and I was thinking maybe my calf muscle got too tight (I haven't stretched them in a while) and is causing the pain. What should I do? Ice it, heat it, stay off of it? Also like I said it only hurts when I walk and do the heal to toe movement. It doesn't hurt to just stand or sit or move my foot. Thanks